Do you want the 'legal' answer, or do you want to make things work?
Panels, the bussbar assemblies, and breakers are tested as a complete assembly. This is why putting a brand "a" breaker in a brand "b" panel can be a code issue. Technically, it's no different with the panel cover.
In addition, panels are required to have various markings- many of which are placed on the cover or dead front. A replacement cover might not have those stickers, thus putting the entire panel out of code compliance.
Indeed, simply repainting a cover might cause a code violation by covering those stickers.
That's the 'legal' answer.
Then there's the real world. omehow a panel face comes up missing at the end of a job, or is somehow damaged, or has far too many breaker KO's removed What are you to do?
Even for a current panel, it's darn near impossible to buy a replacement piece. They'd just as soon have you replace the entire panel. What if the panel is of a discontinued make? Not every panel can be readily replaced.
I will admit that, yes, I have had replacement covers made. I've even had a paint shop mix up some spot-on paint. Technically a code violation, I have no doubt that the covers I made would pass every test at UL -including corrosion testing- and just lack the necessary stickers. I doubt any inspector will ever notice.
My objection to the OP's question lays in his use of the word "makeshift." While I would not object to an "equivalent" replacement, I'm not about to accept (for example) a piece of plywood in place of the original cover.